Investigating the drag sensitivity parameters and lateral dynamic response of a road vehicle in cross-wind flow

Abstract

Recent trends in the automobile industry focus towards enhancing the operating efficiency of the road vehicle. One can achieve this by a combination of increasing the powertrain efficiency, reducing the weight and increasing the aerodynamic performance. The scope of this study is on the latter, i. e. enhancing the aerodynamic performance. Most cars are optimized for minimum drag in idealized conditions, driving engineers to design for the test rather than for to optimize for actual flow encountered in real world conditions. These realistic conditions include (un)steady cross-wind flows encountered by the vehicle, rather than a perfectly aligned flow as is the case in an idealized situation. Different researchers studied the effect of these real world conditions on the performance of a vehicle. Many of these studies focussed upon the vehicle stability rather than the potential to reduce aerodynamic drag. This is because typical drag reducing means (such as radiused edges) tend to have a detrimental effect on the cross-wind stability and comfort of the vehicle. \\ The introduction of the fully electric Tesla Model S created new opportunities within this conception. This 2500 {kg vehicle has an 800 {kg battery underneath the car, which results in a different - relatively flexible - position of the center of gravity, total mass and corresponding mass moments of inertia. As a result it is questioned if this difference in vehicle specifications allow for drag reducing shape modifications on a vehicle which is then still stable during cross-wind flows. After a careful trade-off it was chosen to use the recently launched open-source CFD software suite SU2 in order to find an answer to the following research objective: What design modifications reduce the drag coefficient of a simplified vehicle model which experiences a cross-wind flow, and how does this affect the lateral dynamic performance? An interesting follow-up question on this would be to identify which design variables are (most) sensitive to drag increments in realistic crosswind flows. This document describes the process of solving the research objective within the framework of a Master of Science thesis. A thorough vehicle dynamics study was performed in order to assess the most influential parameters which affect the lateral deviation of a vehicle. Within this study it was found that the cross-wind induced lateral deviation with a longitudinal velocity of 30m/s and a cross-wind flow of 3.15m/s or 6 degrees is roughly similar to the situation where the steering wheel angle is set to 1 degree. This implies that the lateral deviation during cross-wind flows is not much of an issue during steady cross-wind flows of up to 6 degrees. Experiments were designed for three different cross-wind flows; 0 degrees, 3 degrees and 6 degrees cross-wind flow for both conventional and electric vehicles. Here the effects of the following shape modifications have been studied: arrowing the front of the vehicle, tapering the aft of the vehicle, applying a side-window tumblehome angle, varying the front and rear window angle, and varying the A - and C - pillars. The resulting drag coefficients for each configuration has been averaged over the three cross-wind angles. Here it was found that the most important shape modifications for the drag coefficient occur aft of the vehicle. The optimal angles are listed below, where the original angle is shown in parentheses. The resulting cross-wind flow averaged drag coefficient is shown per shape modification. Arrowing angle (0) - 15 degrees: 5% Tapering angle (0) - 15 degrees: 27% Tumblehome angle (0) - 15 degrees: 10% Front window angle (30) - 25 degrees: 2% Rear window angle: (25) - 15 degrees: 16% A-pillar radius: (0.10) - 0.15m: 4% C-pillar radius: (0.15) - 0.15m: 0% When these design modifications are simultaneously applied it was found that the drag coefficient is reduced by 17% for symmetric flow conditions, 30% during 3 degrees cross-wind flow and as much as 43% during 6 degrees during cross-wind flow conditions. This combines into a cross-wind averaged drag reduction of 30%.